I think you need to carefully consider the outcome for this potential second child. Whether ND or NT having a high needs sibling will have a significant impact on their childhood.
How you will cope in the future, whether the possible future child is ND or NT. Do you have the mental, as well as physical, energy and resilience, and the financial stability to meet the needs of both children? You don’t mention your partner, but children with severe needs do place additional pressures on a relationship. All things you need to be sure of before going ahead.
There is no guarantee that if your second has severe needs that they will be similar to your eldest’s needs. They could be totally at odds and you need to be able to cope if your current set up does not work for that child. Or if they are ND/have SEN, what if they are less severe than your eldest, but still need a lot of input- which you will struggle to give, from the sounds of it.
You also need to carefully consider the well-being of your second child if they are NT. Inevitably, their needs will come second to you eldest on a regular basis, even with the best will in the world. My cousins were those children (now both adults)- though in this case the eldest has severe disability due to complications at birth. He has limited communication, essentially a 4, possibly 5, year old in terms of cognitive/emotional development, but a large and physically imposing man who is prone to physically and verbally aggressive outbursts. His younger brother has a very complicated set of emotions surrounding his brother, and his parents- he loves them and can acknowledge that his parents tried their best to do the right thing for them both, but they did not succeed in meeting his needs very consistently. Totally unconsciously much of the time, but sometimes in the full knowledge that he was getting the shitty end of the stick- either because they had few choices, or because his needs were deemed greater, or they couldn’t deal with another melt down/ tantrum/ aggressive outburst as they were frequently physically and/or mentally exhausted.
He has a strained relationship with his parents, they aren’t especially close and he doesn’t want a guardianship/formal role when it comes to the future care of his brother. There is hurt and resentment there. My aunt and uncle are very saddened by this, but nothing changes what has happened.
I know not all siblings end up feeling sad and resentful, but many do. Personally, I would not chose to bring a second child into this situation. I doubt I have the resilience to make it work for both children/us as a family. I would not want a child to experience what my cousin did, and I am not sure I could guarantee that. If you are very sure you can do the absolute best by your current child and any potential children you have, then consider it.